Water quality changes due to urbanization can result in changes in algal assemblages in part due to eutrophication (high nitrogen and/or phosphorus concentrations) and changes in water flow. Most algae are microscopic but large increases in algal abundance can be seen as thick layers or mats on stream surfaces. One group of algae, the diatoms, tends to decrease with degrading water quality as the amount of urban land increases. Blooms of another group, the green algae, may show up as long, thick green strands covering part of all of the rocks on stream bottoms. In addition, some species of blue-green algae (sometimes called "pond scum") that bloom under high nutrient conditions can produce toxins of concern to fish, wildlife, and humans. Algae are a critical part of the food chain in aquatic systems because they are the "primary producers" - organisms that can convert the energy of sunlight, together with certain chemicals, to an energy source for most other organisms. Different groups of algae produce energy sources of varying quality, so changes in the kinds and abundance of algae in a stream can affect many other organisms in the stream.
Data on algal species, their abundances, their pollution tolerance and other environmental preferences, allow computation of metrics for water quality assessment in a manner similar to that used for invertebrates and fish. Examples of metrics computed include chlorophyll, algal biomass, abundance and biomass of different algal groups (diatoms, green algae, blue-green algae, red algae, etc.), number of algal taxa, number of algal cells, percentage of nitrogen-fixing algae, Shannon diversity, and percentage of pollution sensitive and pollution tolerant taxa. Algal sampling can provide a complementary means of assessing water quality to provide a time-integrated picture of changes occurring as a result of urbanization.
As part of the Study of Effects of Urbanization on Stream Ecosystems (EUSE) we collected benthic algae samples from 30 sites during late August to early September 2004. Two types of benthic algal samples were collected as part of the EUSE Study: a quantitative Richest-Targeted Habitat (RTH) sample and a quantitative Depositional Habitat (DTH) sample. Methods are described in Moulton and others (2002). Quantitative samples are collected to provide actual abundances of species found in selected habitats at a site and allow computation of measures or metrics. The RTH sample is intended to represent the habitat with the greatest diversity of species, usually a riffle. The DTH sample is collected from shallow areas along the edges of the stream where sediment has been deposited. In addition to RTH and DTH samples for species identification and abundance, RTH algal samples were subsampled for analysis of chlorophyll and algal biomass.
Data analyses for this Study are currently in progress. Results will be published in 2006-7 in USGS reports and journal articles.
Moulton II, S.R., J.G. Kennen, R.M. Goldstein, and J.A. Hambrook, 2002. Revised Protocols for Sampling Algal, Invertebrate, and Fish Communities as Part of the National Water-Quality Assessment Program. U.S. Geological Survey Open-File Report 02-150, 72 pp.